Detect mutable in immutable class on definition level

[snapshotpl]

Fixes introduced here #2805 works on class usage. But IMO it should be detected on class definitions.
https://psalm.dev/r/74b7779d86

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/74b7779d86

<?php

/** @psalm-immutable */
class Foo {
	public DateTime $datetime;
  
  public function __construct(DateTime $datetime) {
  	$this->datetime = $datetime;
  }
}

Psalm output (using commit 8926f1d):

No issues!

[muglug]

I don’t see the problem here - can you provide an example where purity/immutability promises are broken?

[weirdan]

If Foo is only used externally (is a part of library API) and your tests are not type-checked Psalm wouldn't highlight the issue above.

[muglug]

Yeah, but the same goes for a lot of Psalm's issues.